High-strength hot-formed steel sheet member

Abstract

A high-strength hot-formed steel sheet member exhibiting both a consistent hardness and delayed-fracture resistance, and is characterized in that: the high-strength hot-formed steel sheet member has a prescribed chemical composition; the degree of Mn segregation α (=[maximum Mn concentration (mass %) at the sheet center in the thickness direction]/[average Mn concentration (mass %) at a depth of ¼ of the total thickness of the sheet from the surface]) is less than or equal to 1.6; the steel purity value as defined in JIS G 0555 (2003) is less than or equal to 0.08%; the average grain size for prior γ grains is less than or equal to 10 μm; and the number density of the residual carbides is less than or equal to 4×10³ particles/mm².

Description

TECHNICAL FIELD[0001]The present invention relates to a high strength hot formed steel sheet member, more particularly relates to a high strength hot formed steel sheet member excellent in delayed fracture resistance.BACKGROUND ART[0002]In the field of steel sheets for automobile use, to achieve both lighter weight for improved fuel efficiency and improvement of the impact resistance, there has been growing use of high strength steel sheet having a high tensile strength. However, along with higher strength, the press formability of steel sheet falls, so production of complicated shapes of products has become difficult.[0003]As a result, for example, along with the higher strength of steel sheet, the problem of the ductility falling and fracture occurring at portions with a high working degree and the problem of the springback and wall camber becoming greater and therefore the dimensional precision deteriorating arise. Therefore, it has not been easy to press-form steel sheet having ...

AI Technical Summary

Benefits of technology

[0024]According to the present invention, it is possible to obtain a high strength hot formed steel sheet member having a 1.7 GPa or more tensile strength and able to realize both hardness stability and delayed fracture resistance. The high strength hot formed steel sheet member of the present invention is particularly suitable for use as an impact resistant part of an automobile.

Problems solved by technology

However, along with higher strength, the press formability of steel sheet falls, so production of complicated shapes of products has become difficult.

As a result, for example, along with the higher strength of steel sheet, the problem of the ductility falling and fracture occurring at portions with a high working degree and the problem of the springback and wall camber becoming greater and therefore the dimensional precision deteriorating arise.

Therefore, it has not been easy to press-form steel sheet having a high strength, in particular 780 MPa or more tensile strength, into a product having a complicated shape.

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